An quadrature frequency multiplex signal transmission system, which is generally called to as QFDM (Quadrature Frequency Division Multiplex) modulation or CQFDM (Coded QFDM; "Coded" means channel coding for error correction) modulation, is one of digital modulation techniques which is planed to be adopted for use in digital audio broadcasting (referred to as DAB) by the ITU-R (ex-CCIR) in near future. The details of this technique are described in the contribution document (TG11/3) issued from ITU-R and the Report of the Television Society, Vol. 17, No. 54, pp. 7-12, BCS 93-33 (September 1993). These previous designs will be discussed in the scope relevant to the present invention.
Since one symbol of QFDM is composed of carriers of several hundreds through several thousands, it is possible to carry out interleaving in both the time and frequency domains of the symbol. Because there is no lack of continuous data by applying interleaving, even when a reception had failed for a length of time, the possibility of restoring data is heightened through an error correction process at a receiver section. Similarly the possibility of restoring data through the error correction process at the receiver section can be enhanced, since, even if carriers over a certain range of frequency had failed by selective fadings caused by a multipath, etc., the interleaving can restore the lack of continuous data. The time interval of the frame is defined in accordance with transmission conditions of the frame, so that a required depth of interleaving should be obtained.
There are some examples of the QFDM transmission system as proposed by the DAB, which employ such interleaving in both the time and frequency domains in consideration of the poor receiving conditions of mobile radio communications. This means that the frame consisting of several hundred symbols is altered so that the arrangement of the symbols in conformity with a prescribed rule.
As shown in FIG. 1 a frame comprised of 300 symbols and 448 carriers is constructed in time and frequency directions respectively. The first symbol is a null symbol (zero magnitude for all the carriers) that is used for synchronizing operations in the receiver section and the second symbol is an equalization reference symbol that is used for removing multipath signals. Then comes a symbol comprised of fixed data that is used for controlling signal transmission parameters, which is followed by information data (i.e., symbols of effective data). The interleaving is performed using a RAM (Random Access Memory) that is associated with this frame and is provided at the transmitter section. Using the RAM, a write-in operation is made in a prescribed sequential order, and then a read-out operation is performed in a sequence that differs from the write-in order. FIG. 1 represents an instance where plural audio channels (33 channels) are transferred simultaneously with an effective data period that is divided into 33 equal parts. Non-multiplexed signal transmission is handled in a similar manner.
FIGS. 2 and 3 respectively show block diagrams of a transmitter and a receiver both associated with the above-mentioned conventional transmission system.
In FIG. 2, a 2-bit information data that is input into an input terminal 1 is transformed by a constellation mapper 2 into a four-phase QPSK constellation signal. The constellation in this context means a representation, in a complex vector plane, of the in-phase axis component and quadrature axis component in an quadrature modulation. In case of QPSK, the constellation has concentrically arranged four symbols with equidistant intervals therebetween, as shown in FIG. 15. These constellation signals are collected in a frame as shown in FIG. 1 and are written into an interleaver 3, which is comprised of memories. For simplicity of explanations, the symbol in the constellation signal in each QFDM carrier will be referred to as "modulation symbol of each carrier", while the symbol in the QFDM signals, i.e., the symbol in all the carriers, will be referred to as "QFDM modulation symbol" or may simply referred to as "symbol". In other words, one element in the two-dimensional plane, as shown in FIG. 1, is the modulation symbol of each carrier, while the respective columns correspond to the QFDM modulation symbols. The coherent symbols (null and equalization reference symbols) are inserted into the interleaver 3, where they are read out in conformity with a specific rule. The interleaved outputs are differentially encoded by a differential encoder 4. The differential encoding is a method for transmitting information by a phase difference between two consecutive symbols, thus characterized in that it needs no absolute reference signal. This differential encoding should be performed for each carrier in the QFDM transmission. That is, processing is to be made in such a manner that the differential encoding is carried out for two consecutive symbols along the column direction, as shown in FIG. 1. DAB has proposed mainly this sort of differential encoding. Then the differentially encoded output is transformed from a frequency domain into a time domain for every modulation symbol in an inverse FFT circuit 5. The respective columns, as shown in FIG. 1, are output as time domain waveform for a certain period of time. Note that this output is in general a complex signal. After a guard symbol period, which is used for preventing the inter-symbol interference by multipath is inserted by a guard symbol inserter 6, the complex signal is converted into an analog waveform by digital-analog converters 7a and 7b. This is followed by frequency conversion by the quadrature modulator. This conversion occurs after the analog waveform has been band-limited by LPFs 8a and 8b. The quadrature modulator comprises mixers 10 and 11, a 90.degree. phase shifter 12, a local oscillator 13 and a mixer 14. Taking the complex signals output from the inverse FFT circuit 5 as in-phase axis component I signal and quadrature axis component Q signal, this modulator synthetically modulates them by local oscillator output with a zero phase and that with a ninety degree phase. The output of the quadrature modulator is an intermediate frequency signal (referred to as IF signal). The IF signal is band limited by a BPF 15 (such as a SAW filter), amplified by an amplifier 16, and then frequency converted in the section comprising of a mixer 17 and a local oscillator 18. The resulting signal will be output as a radio frequency signal (referred to as RF signal).
Referring now to FIG. 3 a block diagram of the receiver associated with the above-mentioned transmission system will be explained hereinafter.
The RF signal is input into an input terminal 31 and is band limited at a BPF 32. A desired signal is then selected at a channel tuner that is comprised of a mixer 34 and a variable local oscillator 35. The desired signal has passed through an amplifier 33 and has been band limited at the BPF 32. After being band limited by a BPF 37 (such as a SAW filter), the signal passes through a variable gain amplifier 38 and is detected at a quadrature detector comprising of mixers 39 and 40, a 90.degree. phase shifter 41 and a variable local oscillator 53. This output is equivalent to the I and Q signals at the transmitter section. After being band limited at LPFs 42 and 43, these signals are digitized respectively at analog-digital converters 44 and 45 so as to be converted into complex digital signals. The digital signals are distributed, and one of the distributed signals is fed to an envelope detector 46 so as to be used as a control signal for an automatic gain control (referred to as AGC) amplification. The other distributed signal is fed to an FFT circuit 51 through guard symbol removers 49 and 50, and thus each symbol of the signal in the time domain is transformed into the signal in the frequency domain (symbol associated to each column in FIG. 1). Further, the complex digital symbol as distributed is fed to a sync. signal extractor 47 so as to detect the symbol and frame synchronization using the null symbol and equalization reference symbols. The detected output is input into a timing generator 48 so as to recover clock and timing signals required for respective signal processors.
After the guard period symbol is removed at guard symbol removers 49 and 50, the signal processed by the FFT circuit 51 is decomposed into modulation symbols for respective carriers, and then equalized for each carrier at an equalizer 55 and an equalization reference symbol detector 56. Further, a delay or differential detector 57 detects phase difference information (most of the DAB proposed methods do not generally need this equalization, it is referred to here, however, to clearly differentiate it from the present invention). As stated above, the information is transferred only by the constellation phase difference in the differentially encoded QPSK modulation, this phase difference is detected by this detector. As a general rule the differential detector 57 is made of a simple differential detector. Next, the differential detection output is restored into the initial frame construction by a deinterleaver 58 that carries out the inverse process of interleaving that was performed at the transmitter section. Furthermore, the modulation symbols for respective carriers are demodulated into two-bit data at a constellation demapper 59 which inversely performs the constellation conversion that was processed at the transmitter section.
The recent general trend is toward the digital broadcasting not only of the sound but also the TV signal, which has led to some proposals for using the QFDM also in digital TV broadcasting. On the other hand, since the digital TV broadcasting requires a higher transmission capacity than the DAB method, a modulation form is customarily used with higher transmission efficiency. What is problematic in this system is that the modulation with higher transmission efficiency requires in general better transmission conditions, namely better receiving C/N ratio (carrier to noise ratio). In the DAB method, for example, a quadrature phase shift keying (referred to as QPSK) is used as a modulation form to modulate the respective carriers of the QFDM. The digital TV broadcasting has also proposed a 16-quadrature amplitude modulation (referred to as 16 QAM) and a 64-quadrature amplitude modulation (referred to as 64 QAM) in addition to the QPSK. It should be noted that in any multi-valued modulation form the more the multi-valued level number, the required C/N ratio increases, thereby reducing the service area. Moreover, it is one of the characteristics of the digital broadcasting that negligible geographical difference may cause worse receiving conditions. In some cases, reception might be absolutely impossible. To overcome such a situation, a concept called "graceful degradation" has been proposed. This concept consists of a hierarchical demodulation of such information only as can be received in terms of the receiving conditions of the receiver.
The modulation form that can be used in the QFDM transmission is (multi-valued) quadrature modulation and one similar thereto, namely a QPSK (equivalent to 4 QAM), an N-QAM or an N-phase PSK (N: integer 2 or higher). The PSK larger than the 16-phase level is not generally used because its required C/N ratio that is higher than the 16 QAM. In addition, the QAMs other than 2 QAM and 4 QAM (2 QAM and 4 QAM are equivalent to the 2-phase PSK and the QPSK) is restrained in that it is difficult to differentially encode due to the characteristic of its constellation and that it should be demodulated through the coherent detection.
As has so far been discussed, the DAB transmission system performs a modulation presupposing such differential encoding as the QPSK. In consideration of the digital TV broadcasting, however, it falls short of the transmission capacity in its modulation form with lower multi-valued level such as the QPSK that enables the differential encoding. In consequence therefore it is compelled to utilize such higher multi-valued modulation as is difficult to differentially encode.
Even such a hierarchical transmission as the graceful degradation using a modulation form with higher multi-valued levels only, or using simultaneously plural modulation forms with different C/N ratios required in such a multicarrier transmission as the QFDM, accomplishes nothing unless a stable demodulation operation is systematically guaranteed even under poor receiving conditions. Previous devices made no effort to stabilize the receiving operation as a transmission system except by the utilization of the null symbols.